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Friday, April 5, 2013

Fixing incorrect timezones in Ruby

Timezones can be tricky to manage and sometimes I just get them wrong - like now...

I had been parsing strings representing local times in Seattle and then storing these without an explicit timezone on a machine which uses UTC by default - so they got stored as that time in UTC (not converted to UTC - the exact time string in UTC.

For example: Fri, 05 Apr 2013 14:24:39 (the local time in Seattle) was stored as Fri, 05 Apr 2013 14:24:39 UTC +00:00 which is 7 hours earlier than it should be.

The easiest way to fix this, that I found is to convert back to a string, strip of any record of a timezone and then parse it back into a time, having temporarily set the TZ environment variable to the correct local timezone.

NOTE that this works on UNIX/Mac OS X systems - not sure about Windows...

puts ENV['TZ']         => nil
puts t                 => 2013-04-05 14:24:39 UTC
s = t.to_s
puts s                 => "2013-04-05 14:24:39 UTC"
s.sub!(/\s+UTC/, '')
puts s                 => "2013-04-05 14:24:39"
ENV['TZ'] = 'US/Pacific'
t1 = Time.parse(s)
ENV['TZ'] = nil
puts t1                => 2013-04-05 14:24:39 -0700

Remember to reset the TZ environment variable back to nil

Be very careful about using gmtime and localtime as these methods operate in place. So ALWAYS make a copy of your time before applying those conversions

t            => 2013-04-05 15:25:41 -0700
t1 = t.dup

t.gmtime     => 2013-04-05 22:25:41 UTC
t            => 2013-04-05 22:25:41 UTC
t1           => 2013-04-05 15:25:41 -0700

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